As typical equipments of an image input device, digital cameras are spreading. Concerning this technology, there has been introduced as a feature story in a magazine “Electronics”, Ohm K. K., Vol. 43, January (1998). As a single-plate type CCD sensor to be employed in the digital camera and so forth takes an optoelectric conversion element consisted of a combination of a photodiode and a capacitor, as a pixel, and converts a distribution of light projected on a plane into an electric signal by arranging a plurality of pixels on one substrate. Upon inputting a color image, s kinds of color signals having mutually different spectrum distribution are produced utilizing means, such as s kinds of color filters, dichroic mirror or so forth. As a device construction for inputting s kinds of color signals using a plurality of CCD sensors, respective s in number of CCD sensor may pick-up s kinds of color signals by arranging respective color filters of s in number of CCD sensors, for example.
On the other hand, as device construction for inputting s kinds of color signal utilizing the single-plate type CCD sensor, s kind of color signals can be picked-up by providing s kinds of color filters to each pixels (optoelectric conversion element). As signal conversion means for generating s kinds of color signals in each pixel using the color signal picked-up by the CCD sensor, a dedicated signal processing LSI for processing in a stage of analog signal output from the CCD has been manufactured and marketed. The color signal input by the color sensor us stored in the memory or output externally after conversion into YC signal (luminance Y and color difference signals Cr and Cb), for example.
Here, for data format of the color signal including the YC signal, there is a system defined by standardization of a television broadcasting equipments, established in Recommendation ITU-R BT, 601-5—“Studio Encoding Parameters of Digital Television for Standard 4:3 and Wide-Screen 16:9”, for example, as recommendation of international organization ITU-R (International Telecommunication Union). According to the standardized system, at first, there are provision for the color signal and provision for the image size, and setting of the color signal corresponding to number of pixels is determined. As one example, the color signal is consisted of three kinds (s=3), i.e., the luminance Y and the color difference signals Cr and Cb. The color difference signal can be sub-sampled in equal number of the number of pixels or lesser number than the number of pixels. Namely, concerning four pixels, it can be a format containing four Y, four Cr and four Cb may be set, or, in the alternative, a format containing four Y, one Cr and one Cb may be set. Aggregating these provisions, it is premised to the number of pixels consisting a broadcasting screen image, and the color signal is defined on the basis thereof.
On the other hand, it has been generally known that a degradation component called as return frequency can be generated unless a relationship of a sampling frequency fsample and a frequency of an output color signal fcolor (fsample/2)>fcolor is satisfied on the basis of principle called as “sampling theorem”. Accordingly, after execution of conversion of the color signal and so forth, a frequency filter is provided.
A concept of the color signal format follows a color signal format defined under JPEG (joint photograph expert group) known as compression method of the color image. In this case, number of pixels forming the screen image can be arbitrarily set. However, corresponding relationship between the pixel and the color signal is set by the same setting method of the case if television.
On the other hand, a technology for improving resolution of a single-plate type color sensor has been disclosed in Tamura and Matoba, “High Definition Pixel Interpolation Method by Single-Plate Color CCD Input”, The Institute of Electronics. Information and Communication Engineers, 1998 National Conference Preliminary Report D-11-87 (March, 1998).
The image data is subject to color conversion of the color kind depending upon application. In case of printer, the input signal is converted into a color signal of CMY (cyan, magenta and yellow) and so forth time identifier use as control signal of ink amount. Conventionally, in order to perform color conversion, signal processing is performed on the basis of preliminarily prepared conversion formula or conversion table. Then, in order to realize reproduction ability of color not depending upon equipment characteristics of input device, it has been known ICC specification defined by ICC (International Color Consorium).